fix: skip grouped positionals satisfied by options

[Agentriel]

Fixes #1794.

This changes positional parsing so a positional argument that belongs to a non-multiple ArgGroup is skipped when another member of that group has already been provided on the command line and there is a later positional that can receive the value.

That lets --option1 value bind value to the next positional instead of first assigning it to the grouped positional and then failing validation with an argument conflict. If there is no later positional, parsing still leaves the value on the grouped positional so existing conflict diagnostics are preserved.

Tested locally:

• cargo fmt --check
• cargo test --test builder issue_1794
• cargo test --test builder groups::
• cargo test --test builder positionals::
• cargo test -p clap_builder
• git diff --check

[Agentriel]

GitHub Actions is currently waiting for first-time contributor workflow approval on this PR.

I ran the focused checks locally:

• cargo fmt --check
• cargo test --test builder issue_1794
• cargo test --test builder groups::
• cargo test --test builder positionals::
• cargo test -p clap_builder
• git diff --check

[epage]

Moving to a draft as our contrib guide asks for maintainer buy-in on a proposed solution before moving onto a PR. See also https://epage.github.io/dev/pr-style/#d-issue